The use of halogens for the recovery of precious metals has been reported by several authors as an alternate option to cyanide extraction of gold and silver (The Chemistry of Gold Extraction, Second Edition by John O. Marsden and C. Ian House, Society of Mining, Metallurgy, and Exploration, Inc., 2006, pp. 271-275). In these processes, the precious metals are present, after leaching in the pregnant solution, as complexed halides, such as AuCl4− or AgBr2−. It has been reported that as long as the oxidation-reduction potential (ORP) is maintained at appropriate values, in the range of 800 mV (AgCl/Ag reference), these pregnant solutions under acidic conditions are quite stable.
The recovery of precious metals can be done by contacting these pregnant solutions with activated carbons. These carbons, with specific surfaces as high as 500 to 1000 m2/g, are very efficient at retaining gold and silver and leave a truly barren solution. However, in the case of metal leaching with halogens, particularly with bromine, the halogens tend to form stable halogenated compounds with carbon, thus precluding complete recycling of halogens and creating a disposal problem of halogenated carbon. Under these circumstances, in the course of the development of a new approach for precious metals extraction with halogens as described for example in U.S. Pat. No. 7,537,741, it has been found desirable to find an alternate approach to the classical adsorption of gold on carbon as practiced with the cyanide leaching.
Precipitation of gold on an inert solid from a gold solution is well-known because of the highly hydrophobic nature of this metal (Ibid., p. 413). Gold precipitated over silica is known, as reported by J. A. Eisele et al, U.S. Bureau of Mines, Report of Investigation 7489.